In cellular systems, energy consumption in the user equipment (UE) is one of the most important aspects to take into consideration when introducing new features and standards. Energy consumption is determined by many parameters, which differ between different standards, but the power amplifier (PA) is usually a very significant contributor, especially when the user equipment is operating at high output power.
The PA efficiency, i.e. the relation between the transmitted and the consumed power, can be made high by having the power amplifier operate in a non-linear region. This, however, has the side effect that energy is also emitted in other frequencies than the intended ones, thus causing interference to systems operating at these frequencies.
In order to reduce these unwanted out-of-band emissions, the power amplifier can be made less non-linear, i.e. more linear, by increasing the PA bias, but this again reduces the PA efficiency, and thus it may drastically deteriorate the UE power consumption. In several standards, it is instead allowed in some situations to reduce the maximum transmitted power from its nominal value. The need for this maximum power reduction (MPR) depends on the used signal waveform, which is determined by e.g. the modulation.
In WCDMA systems, cubic metric (CM) is typically used as a measure for how much MPR may be applied for a given waveform. A particular WCDMA uplink signal waveform consists of several components associated with different physical layer parameters such as spreading codes, spreading factors, power levels, and modulation, which all determine the cubic metric of the signal in a non-trivial way.
Cubic metric is a heuristic measure, with some numerical constants chosen so that the allowed MPR in dB can be calculated from the cubic metric for WCDMA uplink signals. The basic principle is that different waveforms should be able to fulfill the dimensioning out-of-band requirement (typically Adjacent Channel Leakage power Ratio—ACLR) using the same PA bias. However, it has been found that the MPR indicated by the calculated cubic metric is often over- or underestimated by a substantial amount, compared to what is really required for a particular waveform and a particular power amplifier.
Cubic metric is based on the relative amount of power for third order distortion. It does, however, not consider the distribution of this power in frequency domain, in particular not the distribution between in-band and adjacent channels. WCDMA uplink waveforms with the same cubic metric often have completely different distributions and thus the cubic metric-based MPR often fails.
WO 2008/077540 has introduced modulation profiles as the basis for significantly improved MPR estimation. Modulation profiles address the weaknesses of cubic metric. Rather than only quantifying the total third order distortion power, the channel distribution for third and higher order products are calculated for every waveform. Each such set of power levels originating from one waveform is referred to as a modulation profile. The modulation profiles can be mapped to MPR in many ways. In this document a simple linear combination of the modulation profile numbers given in dB was proven to give a sufficient accuracy. Thus with modulation profiles it is possible to easily and quickly calculate accurate MPR values for all waveforms and for different power amplifier characteristics. This enables the design of user equipment that can operate with significantly smaller margins on design parameters and therefore with much higher power efficiency.
As mentioned above, the required MPR depends on the parameters for the physical channels in a WCDMA signal. In TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”, 3GPP, Release 8, some or all of the following uplink physical channels are used:                DPCCH, which e.g. carries pilot symbols and power control commands;        DPDCH, which carries data for the transport channel DCH;        HS-DPCCH, which carries feedback information for an associated DL HS-DSCH transmission;        E-DPDCH, which carries data for the transport channel E-DCH; and        E-DPCCH, which carries control information for the associated E-DPCH channel(s).        
The number of permissible combinations of parameters for these physical channels is very large, above 300 000 for a 3GPP Rel. 8 signal.
One problem in the existing WCDMA standard is that the cubic metric calculation is difficult to do on the fly. Instead, a natural solution is to calculate the cubic metric offline for all possible channel configurations, and store the corresponding allowed MPR in a look-up table. A drawback is that the look-up table needs to be very large, since the number of allowed configurations is more than 300000 in 3GPP Rel. 8. With the introduction of multi-carrier transmission, the issue becomes even bigger, since the number of combinations grows even more. Thus, any method that simplifies the MPR calculation would be beneficial.
Another problem is that the cubic metric often does not give an accurate measure of the MPR that is actually required to fulfill the out-of-band requirements. This may result in that the power amplifier bias, and hence the power consumption, must be set unnecessarily high.
When cubic metric-based MPR values are used the look-up table is kept relatively small despite the large number of entries. The reason is that the MPR based on cubic metric is specified as numbers rounded to nearest higher value on a 0.5 dB grid. Thus, only three bits per entry will be required to represent an MPR from the specified range from 0 to 3.5 dB. However, if a more accurate MPR should be used, possibly together with optimal power amplifier biasing, the size of the look-up table will increase dramatically.
Therefore, it is an object of embodiments of the invention to provide a method of determining levels of power reduction for signals transmitted from a mobile communications device that reduces the power consumption of the device while keeping the look-up tables at a reasonable size.